I've sent an email or two about this, and posted on the Steam forums as well as on these DoW forums, but the text in-game still appears through the Steam shift-tab overlay is still present. I searched this in the forums and you mentioned a bunch of stuff but this in one of your known bug lists, so this might need to be another to add to the list.

This happens only when you have the Compendium or the race/powers descriptions opened. We've had this bug since the very first Steam version of Small World from last year.

This is a design limitation coming from the way Valve implements the Steam Overlay. They "hack" directly the game's OpenGL layer. Unfortunately, the descriptions are HTML files living in another layer, floating above.

The workaround is simply to avoid using the overlay while the Compendium is opened.